Internal combustion engines are often provided with a throttle valve within their combustion air induction systems to control combustion airflow rate. The throttle valve is typically in the form of a butterfly valve, having a disk secured to a throttle shaft which is journaled to the generally cylindrical internal wall surface of a throat of a throttle body. The diameter of the disk is only slightly smaller than is the inside diameter of the throat, whereby the throttle plate may block all but a small portion of the air from passing through the throat in an idle condition. Typically, a return spring is used to bias the throttle shaft and butterfly closed with respect to the throat, shutting off almost all the airflow past through the throttle valve.
Provisions are made for adjusting the throttle valve to compensate for variations between particular engines, differences in their operating environments, gradual changes in characteristics due to wear, etc.
Adjusting the almost-closed position of the throttle valve is generally performed by rotating an adjusting screw that abuts the throttle arm. The throttle arm is affixed to the throttle shaft, and is disposed to approach a fixed surface of the engine when closed. The adjusting screw is typically screwed into either the arm or the fixed surface, and abuts the other. By rotating the adjusting screw, the almost-closed position of the throttle valve can be precisely regulated.
While a long thread engagement is not needed in such a low-force application, engagement of at least several threads is desirable to avert cross-threading and stripping of threads. This has necessitated the arm being constructed of metal having a thickness sufficient to accommodate the several threads when the adjusting screw is threaded into the arm.
Alternatively, it is known to threadedly engage an idle adjusting screw with a threaded aperture in a fixed surface disposed such that the end of the screw abuts the arm (U.S. Pat. No. 3,269,713, Beck, 1966; U.S. Pat. No. 4,161,928, Teague et al., 1979; U.S. Pat. No. 4,200,596, Iiyama et al., 1980; U.S. Pat. No. 4,502,436, Bonfiglioli et al., 1985; and U.S. Pat. No. 4,940,031, Mann, 1990). All such idle adjusting screws have a feature to enable rotation (e.g., a head having a screwdriver slot) located at the free end; i.e. the end not abutting the arm. In some configurations, and particularly in engine installations wherein there are many surfaces and pieces of auxiliary equipment near the engine, it is difficult to access such an adjusting screw.
It would be advantageous to provide for an engine throttle idle adjusting screw which is threadedly engaged with a fixed surface, abuts an arm of a throttle shaft for limiting rotation of the throttle shaft and thereby of a throttle plate, and is rotatably adjustable from the abutting end.